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ampere per meter (A/m)

What is ampere per meter (A/m, Oe, oersted)?

The ampere per meter (A/m) is the measure for magnetic field strength, as defined by the International System of Units (SI). It is derived from the SI base units ampere and meter (or metre), both of which are constructed from the standard's defining constants. The ampere is the measure of electric current, and the meter is the measure of length.

When current flows through a conductor, it generates a magnetic field of a given intensity. This intensity is referred to as the magnetic field strength or, in some cases, magnetic field intensity. Magnetic field strength is typically represented as H. The way in which it is measured depends on the shape of the conductor.

When current flows through a straight conductor, the magnetic field radiates out from the conductor at each point along the conductor. The field is typically represented by a series of concentric magnetic field lines that circle the conductor. The magnetic field strength is a measure of a magnetic field line at a specific distance from the conductor.

How to measure magnetic field strength as measured by ampere per meter

The formula used to measure the magnetic field strength of a straight conductor is H = I/2πr, where I is the amount of current in amps and r is the radius from the conductor to the magnetic field line. When the current increases, the magnetic field strength also increases at that particular distance. However, as the distance from the conductor increases, the magnetic field strength weakens.

Magnetic field strength works differently when the conductor is a coil. With a coil, magnetic field strength is generally concerned with the magnetic field that develops through the center of the coil. As with a straight conductor, increasing the current will also increase the magnetic field strength. However, the strength in a coiled conductor is also impacted by the number of coils, the length of the coil and the type of material inside the coil. For example, magnetic field strength increases when coils are added or if the coils wrap around iron rather than air.

Diagram of magnetic field strength measured by ampere per meter.
Magnetic field strength as measured by ampere per meter.

The formula used to measure a coiled conductor is H = IN/L, where I is the amount of current, N is the number of coils and L is the length of the coil in meters. The magnetic field strength in a coiled conductor is directly proportional to the linear current density. If the linear current density doubles, so does the magnetic field strength; if the linear current density becomes 1/10 as great, the magnetic field strength also diminishes by a factor of 10.

In Gaussian units, magnetic field strength is measured in oersted (Oe). However, Oe can be converted to A/m and vice versa, as shown in the following formulas:

1 Oe = 79.577472 A/m
1 A/m = 0.012566 Oe

Magnetic field strength is one of two ways that a magnetic field can be measured. The other way is magnetic flux density, which is represented as B and measured in tesla (T). Magnetic flux density is also referred to as magnetic induction. The relationship between magnetic field strength and magnetic field intensity can be expressed through the formula B = μH. The Greek letter mu (μ) represents magnetic permeability, which is measured in henry per meter (H/m).

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This was last updated in February 2023

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